Frequency modulation stabilization system



April 26, 1949. A. G. CLAVIER FREQUENCY MODULATION STABILIZATION SYSTEMFiled March 20, 1947 2 Sheets-Sheet 1 6cm 6 2 a a A WM WM 0 0 6? B5 6 Wr a w w TED FREQ.

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FREQUENCY MODULATION STABILIZATION SYSTEM Filed March 20, 1947 2Sheets-Shet 2 INVENTOR. ANDRE G. (ZAV/fk Patented Apr. 26, 1949 UNITEDSTATS TENT OFFICE FREQUENCY MQDULATION STABILIZATION SYSTEM ApplicationMarch 20, 1947, Serial No. 735,961

i Glairns.

This invention relates to the frequency stabilization of systems of thetype in which pulses, which may be modulated in a given characteristicin accordance with the instantaneous amplitudes of a signal to betransmitted, are used to frequency modulate the carrier wave.

Various systems, single and multichannel, have been proposed, as forexample, in my copending application A. Clavier, Serial No. 735,960,filed March 20, L947, for Frequency modulation com" munication system,and in which pulses, modulated in given characteristic, are used tofrequency modulate a carrier wave.

An object of the present invention is the provision of means forfrequency stabilizing systems of the type to which this applicationrefers.

The above-mentioned and other features and objects of this inventionwill become more apparent and theinvention itself, though notnecessarily defined by said features and objects, will be bestunderstood by reference to the following description of the invention,taken in connection with the accompanying drawings, in which:

Fig. 1 is a block diagram of a multichannel transmission systemutilizing both time division and frequency division multiplexing, andparticularly illustrated as used for cable transmission;

Fig. 2 is a block diagram of a single channel transmission systemutilizing intermittently frequency modulated waves; and

Fig. 3' is a set of curves used in describing the operation of theforegoing systems.

The transmission system of Fig. 1 is adapted to handle a relativelylarge number of different channels. The channels are treated in separategroups, each group being distinguished from the other by beingtransmitted over a different portion of the entire frequency bandemployed. These groups will be referred to herein as group or frequencychannels. Within each group of frequency channel, multiplexing isaccomplished by time division, that is, each group handles a number ofsignals which are mixed, utilizing the principle of time division, andare then trans-- mitted within the unique portion of the fre quency bandbelonging to said group.

The foregoing will be best understood with reference to Fig. 1 in whichonly a limited number of groups and a limited number of channels areillustrated for the easier understanding of the operation of the system.A plurality of signo.1 sQurc s: H8. are. d vided i to and tr d. as heegrol p Ql1cws;-sources let form roup 2 icrmsrounlil; and i 18 forroupv These signal sources are used to modulate pulses which may bederived from a common pulse generator l9, feeding into a pulsedistributor 20. The pulse distributor 20 feeds the pulses sequentiallyto lines 2l-36. The pulses fed on each of the lines 2I-26 are modulatedin each of the groups by the signals from a separate one of the sources.For example, the pulses fed along line 2! are modulated by the signalsfrom source I, 1, I3. Those pulses fed along line 22 are modulated bythe signals from sources 2, 8, I l. The pulses along the other lines arecorrespondingly modulated.

In the specification and the accompanying claims, the terms modulated ormodulation with respect to pulses or pulse trains are used broadly tosignify the variation of a characteristic of such pulses or pulse trainsin accordance With the instantaneous Values of the signal, and are notlimited to any specific techniques or apparatus for producing thisresult. In this connection, the terms amplitude modulated or amplitudemodulation signify such variations with respect to amplitude.

Since the groups A, B and C are similar in operation and in arrangementonly group A will be described. Referring now specifically to group A,amplitude modulation of the pulses is accomplished in modulators 21-32,the pulses from lines Zip-26. feeding modulators 21-32 respectively andthe signal sources l-B respectively, also respectively feeding themodulators 27-32. These modulators may be any of various wellknown typesand may for example, consist of normally blocked amplifiers Which areunblocked by the pulses. The outputs from modulators 21-32 Which are inthe form of trains of-amplitude modulated pulses are fed into the Amixer 33 where the trains of pulses are interleaved. Similararrangements of modulators in groups B and C are provided and theresultant pulse trains are mixed in the B mixer 34 and the- C mixer- 35.The multichannel amplitude modulated pulse outputs of mixers 33 to 35are then fed to frequency modulators 36:38 respectively Where they areused to frequency modulate the carrier wave energy in oscillators 39, 48and 4|. The frequency bands covered by oscillators 39, 40 and ll differsomewhat so that said bands While adjacent, do not overlap each other,thereby providing for frequency division between'group. channels A, Band C as will be. more fully described hereinafter.

Since, the output. of the. transmission system a; of Fig. l. is; to betransmitted by means of cable,

:r it is preferred to beat down the output of oscillators 39-4! and forthis purpose, use is made of an oscillator 42 of a slightly differentfrequency output than oscillators 39-4l. The oscillator 42 is preferablya highly stable oscillator and for this reason is preferablycrystal-controlled. The outputs from oscillators 39-4! are fed intomixers 43-45 where they are mixed with the output of oscillator 42 toproduce beat output frequencies which are applied from each of themixers 43-45 to the same transmission medium, which may be a cable 46.

Designating the frequency of oscillator 42 as Fe, then the frequency ofoscillator 39 is preferably Fo+AF1, that of oscillator 40 is Fn+AF2, andthat of 4| is Fo-I-AFc. AF1, AF2 and AF3 represent three bands offrequencies preferably adjacent each other but not overlapping. Theoutput of mixers 43-45 is AF1, AF2 and AF3 respectively. For the purposeof center frequency stabilization, each of the oscillators 39-4! istreated separately and in a similar manner. Accordingly only thestabilization of oscillator 39 will be described. In accordance with afeature of the present invention, stabilization is produced by selectingportions of the frequency modulated carrier waves which are notfrequency modulated, that is, the portions between those at which thepulses have produced frequency modulation. For the purpose of selectingsaid portions, the output of oscillator 33 is fed to a pulse controlledamplifier 4 which is periodically blocked by pulses from generator l9during the time when frequency modulated portions of the output ofoscillator 39 occur. Between the time of occurrence of said frequencymodulated portions, energy from the output of oscillator 39 is fed viathe amplifier 41 to a stabilization mixer 48 in which such energy ismixed with energy from the stable oscillator 42. The resultant beatfrequency is fed to an automatic frequency control system 49 which maybe for example, in the form of a discriminator, any deviations from thedesired beating frequencyresulting in a direct current voltage which issent back to the corresponding modulator 36 for stabilization purposes.Similar equipment similarly operating is likewise provided in each ofthe group channels B and C.

An understanding of the operation of the foregoing system may be aidedby an examination of the curves of Fig. 3 wherein W1, W2 and Werepresent a portion of the signals from the signal source l, 2, and 3respectively. These produce at the output of modulators 21, 28 and 29pulses P1, P2 and P3 whose amplitudes vary in accordance with theinstantaneous value of the signals of their channels. The pulses P1, P2and P3 etc. are interleaved in mixer 33 and are then used to frequencymodulate the carrier C (Fig. 3). The carrier C has a given centerfrequency which is modulated by the pulses P1, P2 and P3 to producefrequency modulation at the spaced intervals A1, A2 and As, the extentof the frequency modulation varying in accordance with the amplitude ofthe pulses P1, P2 and P3. The modulated carrier Wave C is applied to theamplifier 41 which is periodically unblocked so that the portions A4 andA5 between the frequency modulated portions A1, A2 and A3 are selectedand fed to the mixer 48 as indicated in curve CI. The wave Cl is thenbeaten with the carrier wave F0 from oscillator 42. The resultant beatfrequency is then used to produce frequency stabilization in theautomatic frequency control system 49. To facilitate synchronization ofthe receiver for a transmi's sion system of the type hereinabovedescribed, the pulses of a given channel, for example channel I, may beprovided with a distinctive characteristic, for example a distinctiveduration, by suitable means, such as a pulse shaper, to enable selectionof these pulses at the receiver, the selected pulses synchronizingdistributors at the receiver for proper separation of the various pulsechannels, the group channels being preferably first separated by bandpass filters.

While I have described frequency stabilization in connection with amultichannel transmission system, it may also be used with singlechannel systems, for example, as illustrated in Fig. 2. Referring now toFig. 2, signals from a source 50 are mixed with pulses from a pulsegenerator 5| in an amplitude modulator 52, which may be similar to thosedescribed in connection with Fig. 1, to produce a train of amplitudemodulated pulses at its output. This train of pulses is then applied toa frequency modulator 53 where it is used to modulate the frequency of acarrier wave produced in an oscillator 54, the output of the oscillator54 then being applied to some suitable transmission medium such as forexample, an antenna 55. For frequency stabilization purposes, the outputof oscillator 54 is likewise fed to a pulse controlled amplifier 56similar to amplifier 41 of Fig. 1, to pass only those portions of themodulated carrier wave output of oscillator 54 which are not modulated.These selected portions of the output are thenapplied to an automaticfrequency control system 51 to control, as indicated by line 58, thefrequency of oscillator 54. The

I automatic frequency control system 51 may be any suitable system, andmay include, for example, a reference oscillator of relatively stablecharacteristics such as for example, a crystalcontrolled oscillator, orsimpler forms of automatic frequency control may be employed.

While I have described above the principles of my invention inconnection with specific apparatus, and particular modifications thereofit is to be clearly understood that this description is made only by wayof example and not as a limitation on the scope of my invention.

I claim:

1. A signal transmission system comprising a. carrier wave source, amodulator for frequency modulating said wave solely at spaced intervalsthereof leaving unmodulated portions therebetween, means for selectingsolely unmodulated portions of the frequency modulated wave after thewave has passed through said modulating means, and means responsive tothe frequency of said selected portions for frequency stabilizing saidsource.

2. A system according to claim 1, further including means for producinga train of pulses, means for amplitude modulating said pulses inaccordance with instantaneous values of the signal to be conveyed, andmeans for applying the amplitude modulated pulses to said frequencymodulator to frequency modulate the carrier wave at spaced intervalsthereof.

3. A system according to claim 2, wherein the selecting means includes acircuit for coupling the output of said frequency modulator to thefrequency stabilizing means, and means controlled by the pulses from thepulse producing means for alternately blocking and unblocking saidcircuit.

l. A system according to claim 3, including means for normally blockingsaid circuit and means for applying said pulses to said circuit to 5 6periodically unblock said circuit, whereby fre- NITE quencystabilization occurs only between the mod- U D STATES PATENTS ulatedintervals of the wave. Number Name Date ANDRE G. CLAVIER. 2, 01,978Bedford May 28, 1940 5 2,296;962 Tunick Sept. 29, 1942 REFERENCES CITED2,377,326 Crosby June 5, 1945 The following references are of record inthe file of this patent:

